This invention relates to an improved valve for use with a normally vented liquid storage tank for ships. The invention is an improvement over the valve shown in U.S. Pat. No. 3,868,921.
The principles of the invention described in U.S. Pat. No. 3,868,921 are sound, and the form of the invention shown there is a good form. However, certain new problems have become important, and many of the aspects of the earlier invention can be improved.
For example, while the ship is afloat, there is a possibility of the main valve becoming inadvertently closed during filling or ballasting. This could be caused by an inexpert operator closing the valve manually under the mistaken impression that he was opening it, or for some other reason. It is important that the pressure in the tank not be so great that it could damage or break the tank. Hence, it is an object of the present invention to provide a by-pass means that prevents the building up of damaging internal pressures in the tank if the main valve is inadvertently closed during filling or ballasting.
When there is a by-pass such as the present invention provides, it is important that this not be a source of leakage, and therefore it is an object of the present invention to provide a by-pass system having a relief valve which is kept closed with sufficient pressure to retain the cargo but does not damage the tank structure if the ship does sink. The main clapper valve can still act as a low-pressure check valve allowing flow into the tank if the ship is underwater, while precluding an accidental closure that would seal the tank to outside pressure. Furthermore, when salvage operations are conducted, it is desirable to have the clapper valve held positively closed. Hence, that also is provided for herein.
The pressure reference chamber shown in U.S. Pat. No. 3,868,921 is open to the atmosphere, and therefore there is a possibility that under certain circumstances the valve might be actuated even though the ship does not sink, or the actuating device might fail to operate upon sinking of the ship, because seawater has corroded parts, so that parts of the valve would seize and remain unmovable. It is therefore an object of the present invention to provide a sealed pressure reference chamber, the internal pressure of which cannot be altered, as by water filling a non-sealed chamber, and yet which provides for closing the valve when subjected to the same general conditions as close the valve shown in U.S. Pat. No. 3,868,921.
Another object is to protect the restricted orifice into the non-sealed chamber, so that it will not become plugged and then fail to admit fluid. Periodic inspections are desirable, but periodic replacement of parts are expensive. Thus, where elastomeric gaskets are used for packing for seals and where other short-life parts are used, maintenance becomes a major factor. So another object of the present invention is to provide a valve capable of satisfactory operation though constructed completely from metal and not requiring any use of elastomers. Moreover, another object of the invention is to provide for simple checking of the operability of the valve.
Another object of the invention is to provide for sure retention of the valve clapper by eliminating vulnerable small fasteners such as pins, cotter pins, or snap rings, and providing instead more reliable structure.
Additional objects and advantages of the invention will appear from the following description of a preferred embodiment, since some of these objects and advantages cannot be explained clearly until the structure of the valve itself has been described.